The argument you two are presenting is the Loot Boxes argument. While I can appreciate it, and do agree to a degree it essentially always boils down to: Exercise self control. Dont be a slave to your own whims.

Lets get a bit more philosophical, without wasting huge chunks of the page. Abracadabra spoiler tab appear mofu! ~ :3

While I myself am no fan of Loot Boxes and that sort of shit (in game shop is fine, but show me what I am getting, dont make it random chance for tons of crap), I also dont like the often used argument of people being "unable to resist" because they have a condition that makes them addicted and what not.
Its all about self control. An adult should be able to exercise certain amount of it, in order for these things to not be an issue.
I myself view the 5FG re-roll gacha as a fun endeavor with minuscule chance of getting your most beloved waifu (if you dont have her yet), if you persevere long enough. As you should not even get the millisecond attachment to the other rainbows that are popping up.
Or not unlike me, who didnt really aim for anyone in particular and just kept rolling until a somewhat useful character popped up, which was at least partially what I was looking for.

As far as the rarity of this thing > I believe this was either a one time only, or maybe like once half a year/year thing, or special occasion thing.
I suppose this will be even rarer than the 50x gacha where you get a guaranteed rainbow.
We`ve seen how many so far ? 3 I believe. Thats fairly rare by itself, so I am sure this 5FG re-roll will be even more so.

Just think of it as an amusing gimmick you can have some fun with, no need to immediately jump to the "sinister evil intent" train of thought.
If it was Nutaku I could buy that argument but the DMM devs have shown us plenty of times they are willing to compensate the fandom for their loyalty to the game with all sorts of free stuff. Yes, sure if you go about it the wrong way, and think you are getting all the rainbows in the world from this, then sure, there is tangible worry to be had, but as I said above... as long as you look at this as an amusing gimmick and exercise self control there is no need to even worry about things like these.

Speaking from personal experience here, I`ll share a personal anecdote with ya lads: I was pretty hooked onto Chain Chronicle and Grand Sphere for a while few years ago, and did some impulsive purchases. Eventually after I wasted enough money my sanity returned, and I did a calm calculation and overwiev of the situation, resulting in myself stopping from more spending and eventually leaving those games entirely.
While similar story could be drawn with FKG, after the first experience I was able to select where and when I spend (the 5K DMM points get your Waifu ticket for example). Eventually after reaching my goal of building certain setup of Girls > this changed half way through so now I have girls that dont really get much of use outside of the Whaleship > I felt like I for myself I gave the devs some support but it was time to stop the spending.
I believe last time I did spend, it was like 5 bucks to get the lowest DMM point bundle because I was missing some to use with my left overs to get ... I dont even know what I got from it heh. But no more big purchase, no random wasting of money and then havin to eat dry bread with Ketchup And since I am not spending anymore, little fun things like this reroll gacha dont really bother me.

Just my two cents, take it or leave it I am not gonna force this onto anyone.